U.S. protected areas and U.S. counties
Hello Xiaoou,
SUMMARIZE
The protected areas database has two attributes that are mostly useful to
us: the land management type of each area and the acreage of each area. So
what we would like to do eventually is to aggregate the acreage in
different management types to county level.
To aggregate or summarize features based on a common category, such as land management type, county, region, etc, you will typically use the Dissolve function or to export just with stats use summarize. You will typically dissolve as the last step of the analysis at which point you can export the table to a csv or excel file for reports and graphing.
For the Union tool, you are correct in you understanding. The -1's will be non-matches where there was no corresponding intersection between the features and therefore no FID to attributes to bring over. This is in the tool documentation for Union under "Usage". Note that any tool in ArcMap you can click "tool help" as well for more details.
UNION
Second, many of the protected areas appear multiple times with different
county names in the post-onion file. Does it mean that each of the areas
covers more than one county?
If you take a look at the image it shows that features will be sliced on their crossing boundaries. You should inspect some examples of repeated land areas, but it is likely that one part of the land area is in County A and the other part(s) are in County B, C, etc.
CALULATE AREA
If so, is there a way to calculate the acreage
of a certain protected in each county?
First create a new field in your attribute table to hold acreage (type double will be fine). In the attribute table, you can right click a field and calculate values all records or selected records in that field. You can select Calculate Geometry and select US Acres from the drop down menu. The Field Calculator calculate algebraic equations and programming expressions based on other fields. Become very familiar with using the field calculator as it is a primary tool in GIS and data manipulation.
To summarize acreage by county, union the features, dissolve by county, state name (to avoid false positives), and land type, then calculate field in acres for resulting table.
Hello Xiaoou,
I am happy to help,
What were the settings you used for this geoprocess? You can open the geoprocessing results window under the Window top menu. Click the session and double click the process to see what your settings were.
According to How Union Works the -1 can be due to gaps. Did you allow gaps in this process?
http://resources.arcgis.com/en/help/main/10.1/index.html#/How_Union_works/000800000010000000/
Below is an example of Union with features within a feature class that overlap. In this case, the area of overlap will be duplicated to maintain all the attributes and areas. To re-create the overlapping features, use the Dissolve tool on the feature class produced by Union, and dissolve by all fields except FID.
Below is an example of the result of executing Union with the Gaps Allowed parameter unchecked. A polygon feature is created that would otherwise be left empty. The "gap" features can be identified by doing an attribute query of all the input feature's FID fields that equal -1.
Intersect as an Option:
You could also use the Intersect tool instead. It would result in only the areas that are covered by both county land and protected land. Empty county area would not be included. The resulting land would in essence be the protected land, but with the county name attribute as well. You could then run a dissolve based on land type and county name & state to get totals for each county. To symbolize the USA map, join this dissolved results table back to your original county boundary data based on county name & state.
